Photorefractive image amplification at extremely low light levels

The authors report measurements of image amplification at extremely low light levels in BaTiO₃ using photorefractive two-wave mixing. Image amplification for signal levels as low as 20 pW with a Fresnel number of 382 was observed. Comparisons of Fourier and direct imaging show that amplification of the Fourier transform of the image provides improved image quality and field-of-view at the lowest light levels. Fourier imaging inherently filters out noise generated from scatterers; consequently, minimum detectable signal levels can be lower. Image structure is viably recovered with far smaller illuminated volumes using Fourier imaging; hence, crystals with smaller apertures may be used. Finally, Fourier imaging is readily implementable and does not require complex optical interferometers or mechanical methods, which reduce signal power as well as noise